1. Field of the Invention
This invention relates generally to gas turbine engines and, more particularly, to steam injection into combustion sections of such engines.
2. Description of Related Art
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of high combustor flame temperatures. Gas turbine engine designers and manufacturers are constantly striving to lower NOx emissions so to meet international, federal, and local air pollution standards. This effort has resulted in the use of lean premixed combustion systems in which fuel and air are mixed homogeneously upstream of the flame reaction region. The fuel-air ratio or the equivalence ratio at which these combustion systems operate are much “leaner” compared to more conventional combustors in order to maintain low flame temperatures which in turn limits production of unwanted gaseous NOx emissions to acceptable levels. Steam injection for providing steam to a combustion zone of the combustor has also been developed for achieving low NOx emissions.
Steam injection increases the mass flow and therefore increases the power output and steam injection ahead of the combustion reaction zone reduces the amount of oxides of nitrogen generated in the combustion process. Steam injection is particularly applicable to marine and industrial gas turbine engines which are often located in environments where steam is readily available.
Aircraft engine derivative annular combustion systems, such as the LM series of gas turbine engines from the General Electric Company, have been developed to incorporate steam injection. U.S. Pat. No. 5,239,816 entitled “Steam deflector assembly for a steam injected gas turbine engine” discloses a steam injection system for injecting steam through a steam injection manifold located at a forward end of a combustion chamber casing and located radially inwardly of a diffuser which discharges compressor discharge pressure (CDP) air into the combustion chamber. Such a design is not available to gas turbine engines which have no room radially inwardly of the diffuser for such a steam injection manifold. Furthermore, locating the manifold and injecting steam aft and downstream of the diffuser can cause aerodynamic interference with the flow stream of the CDP air discharging into the combustion chamber reducing performance of the combustor.
It is highly desirable to have an effective means for injecting steam into a gas turbine engine combustion chamber through a steam injection manifold located aft and downstream of the diffuser with a minimum of aerodynamic interference with the flow stream of the CDP air discharging into the combustion chamber from the diffuser. It is also desirable to provide for a circumferentially uniform or axisymmetrical distribution of the steam injection from the steam injection manifold into the combustion chamber.